Here is the ST-05G 500w stirling engine with free downloadable drawings. I definitely have to build this one sometime. Full CNC plans for it are sold with a creative commons license.....that is, you may reproduce it for any non-commercial use without infringing.

Hi, some of my hot air stuff comes up in the albums, and other places on http://www.model-engineer.co.uk. Look in the Beginers section, the Stirling engine thread is the biggest one on the whole web site. There are other threads also.
Ian S C